I Recently found out about this snazzy vactrol (H11F1) which apparently works just like the kind you can make from an LED and a cadmium sulphide photo-resistor, but without the lag! Cadmium sulphide photo resistors have some delay in response and can take 5 seconds to reach a steady state at low levels of light.
However I've had some problems with the H11F1 optocoupler and was wondering if anyone could enlighten me as to why they are happening?

It's action seems a lot more like a bi-directional transistor; it transmits AC very well with no distortion but when used as an attenuator (as one resistor in a two resistor voltage divider) I got a lot of signal distortion. The control voltage (applied to the LED half of the optocoupler) had to be greater than the signal input peak amplitude or I ended up with flat tops to my waveform, it simply chopped the waveform up rather than attenuating it like you would expect from an LDR&LED vactrol or potentiometer wired as a voltage divider.
Usually I see this when there is some sort of impedence mis-match, like when a transistor tries to draw too much current... but in this case, from everything I've read, it should be behaving exactly like a resistor... but it's not doing so in practice.

Additionally when used in a simple RC low pass filter the resulting waveform was near-perfectly triangular! It has a straight rise and fall unlike the standard exponential charge/discharge curve of a capacitor.

I'll continue with more tests._________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

Figure 2! The bastards! Hiding it in figure 2, it's practically the last figure you'd think to look at.

Can anyone recommend a very clean amplifier? Or a method of doing this which keeps the signal to noise ratio good?
If I have to make all my signals <100mV then I'll have to bring them back up to something decent like 10V P-P, but I'll be amplifying a load of noise aswell _________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

Tests update: on a single rail supply I can use signals of up to 150mV with only minor non-linear distortion. I didn't really take IF into account but I maxed things out at a pretty low voltage so I am guessing I was flooding it with current
This means that I may be able to use signals of 300mV P-P on a dual rail supply, which I didn't really want to have to get into but it might be worth it to make a VCF with these things.
I'll do some more calculations for IF (the IR-LED control current) I was using ... seems like it was about 2.5mA, that maxed things out plenty though.

There's something madeningly unhelpful on that datasheet, the resistance against IF graph has no units on the resistance axis._________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

Wouldn't the same methods that work for OTAs, which as I understand are also very low current devices, work for this as well?

What methods do you mean? OTAs are a bit of an unusual creature and function from currents rather than voltages like a regular op-amp, If I remember rightly; thier output is a current but you simply put it through a resistor to ground and hey presto, you get a voltage accross the resistor changing the resistor value/associated output load can tailor the output voltage range.

In the case of the H11F1, the input voltage must be really low, and is completely separate from the IRLED emitter current. This results in a tiny output voltage and tiny output current which you can't do much with. It needs some great amplification! However if I can work at around 200 or 300mV then the amount of amplification needed is halved or even lower from the gain required for 100mV I was expecting to be working with;

Not too high but nevertheless I'll be amplifying up a bunch of noise along with everything else, I never usually need an amplifier with a gain greater than 2 or 3.
In my experience of using amplifiers with high gain, things get pretty noisey, and not in a good way, in an un-wanted noise sort of way, hence my inquiry about low noise stuff _________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

Here's a single supply VCA which works for DC signals aswell, the output can be maxed out pretty easily. This overcomes the need for additional amplification however at such a low voltage small offset voltages now factor into the equation. My ground-to-50mV input was biased up at the output by about 1V at the maximum gain without distortion. Better op amps could fix this, I used an LM358. Others may require a dual rail supply because they can't get as close to ground as lm358/324s can.

P.S. this was all on a 12V supply and Cv ranged from 0 to 10V but maxed out the amplifier at low voltage.

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fuck confusing circuit diagram programs, pencial and paper is so much easier!

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_________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

Wouldn't the same methods that work for OTAs, which as I understand are also very low current devices, work for this as well?

What methods do you mean? OTAs are a bit of an unusual creature and function from currents rather than voltages like a regular op-amp, If I remember rightly; thier output is a current but you simply put it through a resistor to ground and hey presto, you get a voltage accross the resistor changing the resistor value/associated output load can tailor the output voltage range.

I plead tired misunderstanding as I was confusing tiny currents with tiny voltages

You might get better control by using a current source to drive the LED. At the same time, using a (linear) current source allows you to know exactly how much current is flowing through the LED and thus protect it against meltdown.

Such a current source could be built with an opamp. Wire the LED from the opamp output back to the - input. Ground the + input. Feed the control voltage through a resistor R also into the - input. Whatever current goes through R will go through the LED. The input current is calculated as E / R where E is the control voltage value. E should be supplied from a buffer. Make sure that this current does not exceed the rating for the LED.

Using your transistor method will work, but it's more difficult to know how much current is going through the LED. The current source method I described will allow you to calculate it exactly and give you assurance that you are using the full range that it can handle. In the circuit you show, the amount of current is calculated using the transistor's Hfe or Beta figure. Unfortunately, this is something that varies quite a bit from transistor to transistor even amongst the same type number._________________FPGA, dsPIC and Fatman Synth Stuff

Time flies like a banana.Fruit flies when you're having fun.BTW, Do these genes make my ass look fat?corruptio optimi pessima

I thought you weren't talking to me, Jovianpyx?
Is my control circuit posted above, not a current source? I was lead to believe that it was but from your wording there it seems you suggest that it is not... would you care to elaborate? Seeing as though you seem to be on better terms with me
Thanks for the info about op amps although I'm reluctant to use more op amps, I'm trying to keep my parts count down for use in larger circuits.
Additionally your circuit probably wouldn't work on a single supply, right? I would probably have to bias the non-inverting input up to 1/2 VS._________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

What you have there is a simple single transistor current amplifier, it is not called a "current source". After all, if you put a voltage on a resistor, it passes current through it, so why not call that a current source too?

The sense to calling something a current source is that it can be accurately controlled. Without measuring it, can you say for certain how much current will flow through a transistor collector-emitter circuit as you have drawn?

Yes, It would work from single supply if the + input were biased to 1/2 Vpsu. However, this is something I never recommend because there are other issues, one of them being that the virtual ground would have to be the reference for the CV. I will not attempt to talk you out of single supply.

The main point I was making is that the opamp circuit I described will allow you to know exactly the current flowing through the LED. Since the LED can be burned out with too much current, this is something that I would want to know. Another advantage over the transistor amplifier method is that because you know the current, you can design the circuit to use the full LED current range. In the transistor current amplifier and with a given input CV, how do you know exactly how much current the LED is getting? The fact is you don't and you could easily wind up with a circuit that does not allow the H11F1 to use it's full range. Worse yet, build it again and the current (for a given CV) will likely be different because Hfe is not something that is closely controlled in the mfr process of transistors. Hfe can vary quite a bit from one unit to the next. Using the opamp method, if you make copies of the circuit, the current is highly predictable and will produce more consistently equivalent units._________________FPGA, dsPIC and Fatman Synth Stuff

Time flies like a banana.Fruit flies when you're having fun.BTW, Do these genes make my ass look fat?corruptio optimi pessima

I'm reminded of the famous shakespear quote:
A rose by any other name would smell as soldery.
Current source is the common name used for that transistor circuit. "Meltdown" isn't really the appropriate technical term either as nothing really melts half the time regarding that, any competent engineer can make sure it won't overload the LED, said LED can handle 60mA and 0.1Watts according to the datasheet.

I'm not after a highly accurate circuit, but if I was I'd want to take the forward voltage of the LED into account which can vary by half a volt. With such differences between vactrols, doesn't that make any attempt to garner any degree of accuracy with this vactrol futile?
Acctually, I have measures to compensate for differences between vactrols but most of the time they won't be necessary._________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

I'll also add that my response here is not ONLY for Joe. There are many members of this forum who might read this thread and my response was to make it more complete. Perhaps Joe isn't interested in accuracy, but I dare say others are. What Joe does with his circuit is his business. I'm just offering an alternative that is likely to perform in a way that takes better advantage of the H11F1._________________FPGA, dsPIC and Fatman Synth Stuff

Time flies like a banana.Fruit flies when you're having fun.BTW, Do these genes make my ass look fat?corruptio optimi pessima

Then I'm still mystified as to what a current source is because I keep seeing that arrangement of load & transistor associated with every description of a current source I read._________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

You will not see a simple grounded emitter transistor amplifier circuit there.

Not to throw more fuel on the fire, but it is mentioned there under 2.2.1 Current-stable nonlinear implementation

IMO, Joe is using the term in the technically correct manner, while Scott may be applying a more colloquial restriction in the analog synth context.

To generally place a requirement that it be "accurately controlled" is arbitrary. How accurate is accurate? Accurate depends on the application and design requirements.

Look at the key equation for a bjt: Ic=BIb. (how do you get a beta symbol?)

No voltage, just current. This is what I was taught - a bjt in this configuration sources (or sinks) a current independent of voltage. It is, in the purest sense, a current source. How does that not matter?

And depending on the application, the accuracy may be just fine. Sure, for many synth applications a more accurate current source may be desired, but I don't think that means that this simple bjt configuration shouldn't be called a current source. Tomato tomato.

My point had more to do with the use of Hfe and that Hfe is not strictly controlled during manufacture of transistors and it can vary quite widely even for the same part number during the same production run.

One can then build two of these VCA circuits and get two different output currents with the same input CV. This means that one VCA may close completely where another one may not.

The other aspect of my point was the actual amount of current being passed through the LED for a given CV. Using a common emitter DC amplifier, the amount of current passed through the collector circuit can only at best be estimated and could put the collector current in a zone where the LED is in danger of burning out.

And I realize that Joe doesn't care if it's accurate (or linear and your citation is regarding "Current-stable nonlinear implementation" which states that it is nonlinear). That said, Joe is not the only reader/builder and I am offering an alternative driving method that is linear and is easy to calculate the current._________________FPGA, dsPIC and Fatman Synth Stuff

Time flies like a banana.Fruit flies when you're having fun.BTW, Do these genes make my ass look fat?corruptio optimi pessima

I completely agree with those points and that they're worth mentioning.

I can see that you were suggesting a linear current source as a more accurate alternative, but I can also see how what you wrote could be construed as suggesting that the common emitter wasn't a current source at all and may have distracted from your main points. So I think that was worth clearing up._________________My synth build blog: http://gndsynth.blogspot.com/

And yes, many synth DIY people would probably say that the common emitter current amplifier is not a current source. I'll take Wiki's word for it. But after reading it several times, for me, the wiki description is obscure enough that I'm not really sure that Joe's circuit complies with the definition. I do wish they had put a schematic there to show exactly what they mean, tracing currents and showing the maths so that it's obvious. One of the many wiki-flaws that exist.

Can you add any additional information that might clear that up for me (and other readers)?

My understanding (which may be flawed) is that a current source involves a sort of regulation action to provide a constant current regardless of internal impedance changes. I don't fully understand how that works in a common emitter amplifier. Probably some "physics" thing about a transistor... Does this work over the entire operating range of a transistor or is the current sourcing capacity something that is restricted to a specific parameter space?

Perhaps the Synth DIY resistance to calling the common emitter current amplifier a "current source" is the Hfe problem I noted. As in while it may be technically termed a "current source", it is not always particularly useful as a current source - examples being given in my posts above that the results will vary from build to build - a bane of the SDIY builder._________________FPGA, dsPIC and Fatman Synth Stuff

Time flies like a banana.Fruit flies when you're having fun.BTW, Do these genes make my ass look fat?corruptio optimi pessima

Ha, well I've been an EE much longer than I've been involved in synth DIY, so I guess that's why my perspective may be a little different. The linear current source that is typically used in synths is a beautiful thing (I'd never seen it before I started investigating synth circuits and when I realized what it was doing I thought it was pretty cool). But I still tend to think of "current source" in the most fundamental sense.

It's been a while, but this is my understanding:

Current source is the dual of voltage source.
In the simplest terms, an ideal voltage source produces a specific voltage regardless of the current. The current will adjust depending on the load impedance to maintain the voltage.
An ideal current source produces a specific current regardless of the voltage. The voltage across a current source will adjust depending on the load impedance to maintain the current.

An open circuit on a voltage source is fine, specific voltage with zero current, but a short results in infinite current.

A short circuit on a current source is fine, specific current with zero voltage, but an open results in infinite voltage.

Practically, there are obviously limitations to this. If you exceed a voltage sources current capacity, the voltage will drop. If you don't maintain enough voltage on a current source, the current will drop.

I think you guys might be getting it a little backwards when you say regulation acts against internal impedance changes. It acts against external impedance changes.

If you take an NPN transistor in the common cathode configuration with a constant base current, as long as the transistor remains in the active region the collector current will remain constant (as given by Ic=BIb) regardless of Vce (you can see this in the characteristic curve of a BJT). If you have a varying load impedance on the collector, as that impedance increases, Vce will drop (internal impedance of the transistor drops) to maintain the same current. As the load impedance decreases, Vce will increase (internal impedance increases). Of course if Vce drops to Vbe, you enter saturation and it will no longer act as a current source.

In additional tests I acctually got a better response, when configured as a VCF, using a resistor in series with the IRLED and no transistor. Just feeding the CV straight into the 10k resistor. this might be impractical for some applications as most things seem to expect a 100k input impedence, 10k is an order of magnitude lower.

To summarize so far: if you want a simple circuit with variations, go with my circuit. If you want everything to sound sterile and uniform, go with Jovianpyx's._________________As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"

Time flies like a banana.Fruit flies when you're having fun.BTW, Do these genes make my ass look fat?corruptio optimi pessimaLast edited by JovianPyx on Thu Aug 29, 2013 7:24 am; edited 2 times in total

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